Fluorescent somatostatin

ABSTRACT

Light-emitting compounds of the formula: ##STR1## where R 1  is a light-emitting moiety and R 2  is a somatostatin-based peptide featuring a first amino acid sequence --Phe--Phe--Trp--Lys--Thr-- (SEQ ID NO:1) or --Phe--Phe--D--Trp--Lys--Thr--. The peptide is linked at a first amino acid position to (C--X), and the light-emitting compound exhibits substantial biological activity in the presence of a receptor having affinity for somatostatin-based peptides.

BACKGROUND

This invention relates to light-emitting peptide compounds.

Peptides may be complexed with detectable "labels" and used, for example, to monitor peptide, cytokine, drug, and hormone receptors at the cellular level. In a typical application, the labelled peptide is placed in contact with a tissue or cell culture to facilitate binding to an available receptor. Once bound, the label can then be detected, allowing properties such as receptor distribution or receptor binding kinetics to be monitored.

Peptides are typically labelled with radioactive elements such as ¹²⁵ I or ³ H. In this case, emission of high-energy radioactive particles is monitored using standard γ-ray detectors, thereby allowing detection of the label. While detection techniques for ¹²⁵ I and ³ H are well-known, radioactive compounds by nature have limited half lives, and are often both toxic and expensive. Moreover, current detection technology makes it difficult or impossible to detect radioactive probes in real-time, thereby precluding study of kinetic processes.

Somatostatin is a particularly desirable peptide to label and use to monitor cell receptors, as this compound exhibits biological activity in a number of organs, such as the brain, pituitary and endocrine systems, and the kidneys. In particular, somatostatin, when bound to an outer membrane receptor, appears to cause release of hormones and secretory proteins capable of inducing multiple physiological effects (Yamada et al., Proc. Nat. Acad. Sci., Biochemistry, Vol. 89, p. 251-255 (1992)). Somatostatin is synthesized in the hypothalamus and other regions, has a cyclical configuration, and exists naturally as either a 14-residue (i.e., somatostatin-14) or 28-residue (i.e., somatostatin-28) peptide. Both forms share the same 14 amino acids at the carboxy terminus, have similar affinities for the same receptor, and appear to be equally potent in producing a biological response (Maton et al., "Basic and Clinical Aspects of Neuroscience", Springer Sandoz Advanced Texts, Vol. 4, pp 55-63 (1985)).

SUMMARY

In general, in one aspect, the invention features a compound of the formula: ##STR2##

where R₁ is a light-emitting moiety and R₂ is a somatostatin-based peptide comprising a first amino acid sequence --Phe--Phe--Trp--Lys--Thr-- (SEQ ID NO:1) or --Phe--Phe--D--Trp--Lys--Thr--. The peptide is linked at a first amino acid position to (C--X) which, in turn, is selected from the group including C═O, C═S, CH(OH), C═C═O, C═NH, CH₂, CHOH, CHOR, CNR, CH--R, CR₃ R₄, and COR₃ OR₄ where R, R₃, and R₄ are alkyl moieties or substituted alkyl moieties. Preferably, the compound exhibits substantial biological activity in the presence of receptors having affinitities for somatostatin-based peptides. The compound may also be in the form of a pharmaceutically acceptable salt or complex thereof.

In preferred embodiments, the first amino acid sequence, --Phe--Phe--Trp--Lys--Thr-- (SEQ ID NO:1) or --Phe--Phe--D--Trp--Lys--Thr--, is featured at residue positions 7-11 in the somatostatin-based peptide. Preferably, the first amino acid position bound to (C--X) is included in the N-terminus of the somatostatin-based peptide.

In other preferred embodiments, the somatostatin-based peptide further includes the amino acid sequence --Ala--Gly--Cys--Lys--Asn--Phe-- (SEQ ID NO:2) or, in other embodiments, the amino acid sequence --Phe--Thr--Ser--Cys-- (SEQ ID NO:3). In still other embodiments, the peptide includes both sequences; for example, the somatostatin-based peptide may be somatostatin-14 or D-Trp⁸ -somatostatin-14. Preferably, the sequence --Ala--Gly--Cys--Lys--Asn--Phe-- (SEQ ID NO:2) is included in the N-terminus of the somatostatin-based peptide, and the --Ala-- residue is attached to the (C--X) bond. The amino acid sequence --Phe--Thr--Ser--Cys-- (SEQ ID NO:3) is preferably included in the C-terminus of the somatostatin-based peptide.

In other preferred embodiments, the light-emitting moiety is selected from the group including Bodipy, fluorescein, FTC, Texas red, phycoerythrin, rhodamine, carboxytetramethylrhodamine, DAPI, indopyras dyes, Cascade blue, coumarins, NBD, Lucifer Yellow, propidium iodide, and derivatives thereof.

In still other preferred embodiments, the (C--X) bond is either C═O or C═S.

In another aspect, the invention provides a method for labelling a receptor having an affinity for a somatostatin-based peptide by contacting the receptor with one or more of the compounds described above.

In yet another aspect, the invention provides a light-emitting compound of the formula: ##STR3##

wherein R₁ is a light-emitting moiety and R₂ is a somatostatin-based peptide including a first amino acid sequence --Phe--Phe--Trp--Lys--Thr-- (SEQ ID NO:1) or --Phe--Phe--D--Trp--Lys--Thr--. In this case, the peptide is linked at a first amino acid position to (C--X), and the light-emitting compound is selected from secondary compounds having relatively low biological activities.

Most preferably, the selection process is accomplished by exposing the light-emitting compound to a receptor recognizing somatostatin-based compounds, and then selectively isolating light-emitting compounds exhibiting substantial biological activities. For example, the preferred compounds may be selected using high-pressure liquid chromatography ("HPLC"). As before, in this embodiment, the first amino acid sequence for this compound, --Phe--Phe--Trp--Lys--Thr-- (SEQ ID NO:1) or --Phe--Phe--D--Trp--Lys--Thr--, is featured at positions 7-11 in the somatostatin-based peptide.

By "somatostatin-based compound" is meant a peptide which includes somatostatin, fragments of somatostatin, or derivatives thereof. Somatostatin-based peptides may be peptides whose sequences differ from somatostatin's wild-type sequence by only conservative amino acid substitutions. For example, one amino acid may be substituted for another with similar characteristics (e.g., valine for glycine, arginine for lysine, etc.) or by one or more non-conservative amino acid substitutions, deletions, or insertions which do not abolish the peptide's biological activity. Other useful modifications include those which increase somatostatin's stability. The peptide may contain, for example, one or more non-peptide bonds (which replace a corresponding peptide bond) or D-amino acids in the peptide sequence. In a particular example, as described above, the somatostatin-based peptides may be somatostatin-14 or D-Trp⁸ -somatostatin 14.

By "substantial biological activity" is meant the compound preferably binds to a receptor having an affinity for a labelled somatostatin-based compound which is at least 0.25% of that of the corresponding unlabelled somatostatin-based compound. More preferably, the receptor affinity for the labelled somatostatin-based compound is at least 1.0% of that of the corresponding unlabelled somatostatin-based compound. Receptor affinity in this case can be determined using known methods, such as by measuring the K_(d) for the receptor/peptide interaction.

By "relatively low biological activity" is meant a compound which lacks substantial biological activity.

The invention has many advantages. In a general sense, compounds containing light-emitting dyes rather than radioactive labels are advantageous during the synthetic, detection, and disposal processes. For example, when compared to radiolabelled compounds, fluorescent somatostatin is relatively safe and non-toxic, thereby allowing it to be synthesized and used without employing special laboratory procedures. Similarly, following use, fluorescent somatostatin may be easily disposed, whereas disposal of radioactive compounds is both time-consuming and costly. In addition, fluorescent markers for somatostatin receptors are stable and may be stored for extensive periods of time, while radioactive markers by nature undergo radioactive decay and thus have a limited lifetime (for example, the half-life for ¹²⁵ I is 60 days).

Use of somatostatin in the labelled compound is also advantageous. As described above, somatostatin exhibits biological activity in organs such as the brain, pituitary and endocrine systems, and the kidneys, and can therefore be used as a probe to investigate a large number of different cell types. In addition, this peptide has a relatively simple structure (14 amino acids) and can be synthesized and isolated with standard, well-known techniques.

During typical experiments, fluorescent markers for somatostatin receptors emit optical signals, and thus may be monitored by eye or with the aid of external optical detectors. In this way, the fluorescent peptides obviate the need for secondary detection steps sometimes used for radiolabelled compounds or incubation with secondary labeled compounds. Detection of optical radiation is, in general, relatively simple and cost-effective compared to detection of radioactive particles (e.g., γ particles); conventional CCDs or light-sensitive cameras can therefore be used without modification for this application.

In addition, because of their high optical emission rates and well-characterized emission cross sections, fluorescent markers attached to somatostatin receptors can be used for real-time, quantified imaging of a number of dynamic biological phenomena, such as kinetics associated with receptor binding. The compounds can also be used for static processes, such as monitoring peptide distribution within a cell. Somatostatin receptors marked with fluorescent peptides may also be used in flow cytometry, confocal microscopy, fluorescence polarization spectroscopy, and any other techniques exploiting the optical detection of fluorescence or photoluminescence.

Other advantages and features of the invention will become apparent from the following detailed description, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a somewhat schematic drawing of the chemical structure of fluorescent peptides according to the invention;

FIGS. 2A and 2B are somewhat schematic drawings of the chemical structures of, respectively, Bodipy-labelled D-Trp⁸ somatostatin and fluorescein-labelled D-Trp⁸ somatostatin;

FIG. 3 is a plot showing the amount of specific binding of the invention's fluorescent peptides as a function of their concentration; and

FIG. 4 is a photograph showing a Chinese hamster ovary cell containing receptors labelled with fluorescein-labelled somatostatin.

DETAILED DESCRIPTION

Referring first to FIG. 1, in a preferred embodiment, a fluorescent peptide 10 according to the invention includes a light-emitting moiety 12, such as a fluorescent dye, linked via a --(CX)-- bond to a peptide moiety 14. The peptide moiety 14 includes amino acid residues at the 7-11 positions of the somatostatin peptide, which itself is composed of the following sequence: --Ala--Gly--Cys--Lys--Asn--Phe--Phe--Trp--Lys--Thr--Phe--Thr--Ser--Cys-- (SEQ ID NO:4). Thus, as indicated in the figure by the region 18, the peptide moieties include amino acids --Phe--Trp--Lys--Thr--Phe-- (SEQ ID NO:5).

In order to retain substantial biological activity and affinity for somatostatin receptors, the peptide moiety is attached to the --(CX)-- bond at the first amino acid residue (indicated by the region 16) at the N-terminus. For example, in a preferred embodiment, the peptide moiety is attached via a Nα-Ala1 bond to an acyl group. The light-emitting moiety, in turn, is preferably covalently linked at any available position to the --(CX)-- bond. This bond may include groups such as C═O, C═S, CH(OH), C═C═O, C═NH, CH₂, CHOH, CHOR, CNR, CH--R, CR₃ R₄, and COR₃ OR₄ where R, R₃, and R₄ are alkyl moieties or substituted alkyl moieties.

Fluorescent peptides of this type have amino acids which are available for binding to somatostatin-recognizing receptors, thereby enabling the peptide to be used for labelling purposes. Once bound to an available receptor, the attached light-emitting moiety retains optical properties similar to those of the un-bound light-emitting molecule. In this way, the fluorescent peptide can bind to the corresponding receptor and emit light following absorption of an incident optical field, and thus serve as a marker for the somatostatin receptor. This allows the receptor to be "labelled" and permits investigation, for example, of peptide/receptor interactions. In particular, labelled peptides participating in receptor/peptide interactions can be monitored to determine the location of receptors in cell or tissue samples, and additionally allow quantification of receptors, determination of the receptor affinity for various ligands, or the identification of various populations of cells.

Somatostatin-based peptides may be synthesized using techniques known in the art, extracted from natural systems, or obtained from commercial sources (e.g., Peninsula, Neosystems, Sigma, and BASF). A list of somatostatin analogs which may be used is described, for example, in "Somatostatin", Weil, Muller, and Thorner (eds.), (1992), the contents of which are incorporated herein by reference. Of particular relevance is the chapter "Somatostatin Receptors in the Central Nervous System", by J. Epelbaum. Typically, the peptide is either purchased or synthesized using conventional solid-phase synthetic techniques. Preferably, the peptide is substantially pure, meaning that it is at least 60% by weight free from the other compounds with which it is naturally associated.

In general, any dye, porphyrin, fluorophore, or other light-emitting compound may be complexed with the somatostatin-based peptide. As described above, in preferred embodiments, the light-emitting moiety is selected from the group including Bodipy, fluorescein, FTC, Texas red, phycoerythrin, rhodamine, carboxytetra-methylrhodamine, DAPI, indopyras dyes, Cascade blue, coumarins, NBD, Lucifer Yellow, propidium iodide, and derivatives thereof. The synthesis and structures of several dyes which may be used are described in U.S. Pat. Nos. 5,248,782; 5,274,113; and, 5,187,288, the contents of which are incorporated herein by reference.

Once the desired peptide is obtained, fluorescent peptides having high biological activities are made by attaching the light-emitting moiety to the first amino acid position of the somatostatin-based moiety. In general, this reaction is carried out by modifying a functional group on the peptide, most typically a thiol or amine group, so that this moiety may be easily attached to the light-emitting compound. Reactions for such modifications are described in, for example, "Handbook of Fluorescent Probes and Research Chemicals--5th Edition" by Richard P. Haugland (1992), the contents of which are incorporated herein by reference. In general, thiols react with alkylating groups (R'--X) to yield relatively stable thiol ethers (R--S--R'), with the leaving group X preferably being a halogen (e.g., Cl, Br, I) or similar leaving group. In particular, the most common reagents for derivatization of thiols are haloacetyl derivatives. Reaction of these reagents with thiols proceeds rapidly at or below room temperature in the physiological pH range. The general chemical properties of the dyes used in the fluorescent peptides shown in FIGS. 2A and 2B (i.e., succinimidyl esters) are, for example, described in "Set 5" on page 6 of this manual.

The conditions used to modify amine moieties of the desired peptide will depend on the class of amine (e.g., aromatic, aliphatic) and its basicity. Aliphatic amines, such as the α-amino group of lysine, are moderately basic and reactive with acylating reagents. The concentration of the free base form of aliphatic amines below pH 8 is very low; thus, the kinetics of acylation reactions of amines by isothiocyanates, succinimidyl esters, and other reagents is strongly pH-dependent. Although amine acylation reactions should usually be carried out above pH 8.5, the acylation reagents degrade in the presence of water, with the rate increasing as the pH increases. Therefore, a pH of 8.5-9.5 is usually optimal for modifying lysines. The α-amino function of the amino terminus usually has a pK_(a) of ˜7, thereby allowing it to be selectively modified by reaction at neutral pH.

In general, reactive groups on the light-emitting moiety, such as unsaturated alkyl groups, will react with the modified peptide to form a dye/peptide bond. The chemical structure of the light-emitting moiety may affect the synthetic route used to synthesize the fluorescent somatostatin analog. It may be necessary, for example, to modify the light-emitting moiety so that it includes a reactive group prior to exposing this moiety to the desired peptide.

The chemistry used to synthesize the fluorescent peptide does not vary significantly depending on the exact structure of the somatostatin analog. Thus, the general procedure outline below may be used for most somatostatin-based peptides. Moreover, derivatives of somatostatin have been found to be especially biologically active, and may be used in the fluorescent peptides described herein to label somatostatin receptors. For example, a preferred compound is the analog D-Trp⁸ -somatostatin 14 (Ala--Gly--Cys--Lys--Asn--Phe--Phe--D--Trp--Lys--Thr--Phe--Thr--Ser--Cys) which is about eight times more potent than somatostatin, most probably due to this compound's enhanced resistance to enzymatic degradation. (Rivier et al., Biochem. and Biophys. Res. Comm. Vol. 65, No.2, pp 746-751 (1975)). Attachment of this peptide to a light-emitting moiety is described in detail in the Examples provided below.

Once synthesized, the resulting complex is purified, preferably using a column method such as HPLC, and then eluted. Collected fractions are then screened using analytical methods, such as HPLC, to determine if adequate biological activity is present. Fluorescent somatostatin analogs having adequate biological activities are selected by first exposing these analogs to somatostatin receptors; compounds binding effectively to these sites are then isolated from relatively inactive fluorescent peptides. In general, this selection process can be performed using standard techniques, such a column chromatography or other analytical techniques known in the art. The selection process is designed to allow maintenance of the compound's pharmacological binding, and thus allows only the dye/peptide compounds exhibiting substantial biological activities to be separated from relatively inactive compounds.

Referring now to FIGS. 2A and 2B, in preferred embodiments, the light-emitting moiety is attached via an amide moiety to a somatostatin analog containing a D-tryptophan residue at the 8 position. In this case, the peptide has the structure --Ala--Gly--Cys--Lys--Asn--Phe--Phe--D--Trp--Lys--Thr--Phe--Thr--Ser--Cys-- and is attached to an amino acid residue at the 1 position to the amide linkage which, in turn, is covalently bonded to a linear (FIG. 2A) or cyclical (FIG. 2B) alkyl group of the light-emitting moiety. The compound shown in FIG. 2B, Nα-fluoresceinyl D-Trp⁸ !somatostatin-14, absorbs light in the range of 442-488 nm and features an emission spectrum centered around 530-546 nm. The synthesis of a compound having a similar stucture is described in detail in the Examples provided below. The compound shown in FIG. 2A, Nα-Bodipy D-Trp⁸ ! somatostatin-14, absorbs light in the range of 460-488 and emits light at wavelengths centered around 530 nm. Like fluorescein, Bodipy is a succinimidyl ester, and thus synthesis of this compound is similar to that described in the Examples, with the appropriate steps being used to attach the amide linkage to the Bodipy's unsaturated ring structure.

Referring now to FIG. 3, fluorescent peptides having the chemical structure shown in FIGS. 2A and 2B exhibit comparable, dose-dependent binding to somatostatin receptors when compared to radiolabelled somatostatin peptides. The graph shows the binding of fluorescein and Bodipy-labelled D-Trp⁸ ! somatostatin-14 as determined by percent inhibition of the binding of ¹²⁵ I-tyr¹¹ ! somatostatin 14 to somatostatin receptors. The concentrations at which 50% of the binding is inhibited (i.e., the IC₅₀) is 1.2 nM for the Bodipy-labelled compound, and 3.2 nM for the fluorescein-labelled compound, as compared to 0.26 nM for the unlabelled somatostatin.

Fluorescent somatostatin compounds selected to have high biological activities can be used in a number of applications. During most applications, the fluorescent somatostatin compound is first contacted with the sample of interest. The labelled analog is then incubated with the cell or tissue culture for a select time period and allowed to interact with the somatostatin receptor; if necessary, excess, non-specifically bound somatostatin is washed away.

Once bound to the desired receptor sites, the labelled sample is imaged using standard techniques known in the art. In this way, small-scale features in the cell which normally would be difficult or impossible to detect may be imaged. For example, this allows visualization of intercellular receptor sites. Conventional microscopy methods, such as fluorescence or confocal microscopy, may then be used to optically excite and then detect emission from the labelled receptors. Other imaging techniques which can be used with the fluorescent somatostatin-based compounds include atomic force microscopy, fluorescence polarization spectroscopy, and fluorimetry.

Because the labelled peptides, once excited, emit optical radiation, standard optical detectors can be used during the imaging process. For example, detectors such as charge-coupled devices (CCDs), diode arrays, video cameras, or any other light-sensitive detector can be used in combination with the magnifying and optical imaging systems described above.

Optical radiation emitted from the fluorescing moiety can be easily and rapidly detected, allowing the fluorescent peptides to be used to monitor real-time receptor/peptide interactions. In this way, the compounds permit study of kinetic processes in the cell. Other techniques which can use to advantage the labelled somatostatin peptides include flow cytometry, cell sorting, tumor marking, competitive binding assays for drug screening, fluorescent immunoassays, and other in vitro experiments involving compound labelling according to techniques known in the art.

The following Examples are used to more particularly point out the synthesis, selection methods, and use of fluorescent somatostatin analogs having high biological activities.

EXAMPLES

1. Fluorescent Labeling of D-Trp⁸ -Somatostatin

1 mmole NHS FTC (an isothiocyanate analog of fluorescein; Molecular Probes) was first diluted in 200 ml of acetonitrile (Pierce) and then incubated with D-Trp⁸ -ss1-14 (0.6 mmole; Neosystems) in a final volume of 1 ml Borate/Phosphate Buffer (50 mM/50 mM) at pH 6.5 for 3 hours at 4° C. The solution was purified on an HPLC C18 column (10 mm×250 mm, Ultrosphere, ODS, Beckmann Instruments) and eluted in 0.1% TFA with a linear gradient of acetonitrile from 20% to 70% during 100 minutes at a debit rate of 1 ml/min. Elution of the compound was monitored by observing optical density profiles at 213 nm.

Collected fractions were screened by analytical HPLC using both UV and fluorescence detection, with the excitation wavelength being 338 nm, and the emission wavelength at 425 nm, and then lyophilized.

Once synthesized, fluorescent peaks were monitored, with the compound then being subjected to Edmann's degradation to determine whether the α amino function present on the amino terminal amino acid was free or blocked. The first and largest peak was found to be unchanged when the compound was exposed to phenylisothiocyanate, and therefore corresponded to FTC- Ala1!-D-Trp⁸ -ss. The amino acid composition of this fragment was assessed by quantitative amino acid analysis after acidic hydrolysis in vacuo (6N HCl, 110° C., 18 h) and carboxypeptidase Y digestion (6 U/0.3 mmole, 37° C., 48 h).

The site of attachment between the NHS fluorescein and the somatostatin analogue was confirmed to be at the N-terminus, and was identified as Nα-Ala1. Isolation of this peak yielded a compound with a molar ratio of NHS fluorescein to D-Trp⁸ -somatostatin of 1:1. FTC- Ala1!-D-Trp⁸ -somatostatin was evaluated to be pure as indicated by a single elution peak from reverse phase HPLC. The compound was freely soluble in water or aqueous buffer, and was stable if protected from light and maintained in a lyophilized form at 4° C.

2. Pharmacological Binding

Competition studies were carried out with radiolabelled peptides to test the receptor-binding efficacy of FTC- Ala1!-D-Trp⁸ -somatostatin. Here, the competition studies using the fluorescent peptide and ¹²⁵ I somatostatin were performed on purified rat brain membranes according to well-known methods known in the art. Membranes were incubated with 0.1 nM of radiolabeled somatostatin in the presence of varying concentrations of FTC- Ala1!-D-Trp⁸ -somatostatin in 50 mM Tris HCl pH 7.5 containing 0.2% bovine serum albumin. The reaction was carried out for 20 minutes at 22° C. and stopped by addition of ice-cold buffer. Membranes were then subjected to immediate filtration over Gelman™ filters (Millipore™) under vacuum. They were then washed and their radioactivity concentration measured using a standard gamma counter (LKB, Inc.).

Data are expressed as the percentage of specific binding of the radioligand in the absence of fluorescent competitor. IC₅₀ values were obtained from plotting individual values using the EBDA/Ligand program. Referring again to FIG. 3, the fluorescent analogue of somatostatin displaced the specific ¹²⁵ I-somatostatin binding in a dose-dependent manner. The IC₅₀ value for unlabeled D-Trp⁸ -somatostatin was 0.26 nM; the corresponding value for FTC- Ala1!-D-Trp8 somatostatin was 3.2 nM.

3. Confocal Microscopic Visualization of Somatostatin Receptors: Internalization of FTC- D-Trp⁸ ! Somatostatin in CHO Cells Expressing sstr1 or sstr2a Receptors

Cells incubated with the fluorescent peptide were monitored using standard confocal microscopy methods to determine the locations of somatostatin-recognizing receptors. The microscope used was manufactured by Leica. Referring now to FIG. 4, in these experiments, incubation of Chinese hamster ovary (CHO) cells stably transfected with cDNA encoding sstr1 or sstr2 somatostatin receptor sub-types with 10 nM FTC-labelled somatostatin for 45 minutes at 37° C. resulted in an intense fluorescent labeling of the CHO cells. The presence of light-emitting portions of the cells, monitored using serial confocal microscopic optical sectioning, revealed that labelling was intracytoplasmic and in the form of small endosome-like particles. In both types of transfected cells, the sizes of these fluorescent particles were found to increase with time, while the number of particles decreased with time; this suggests a fusion of the endocytic compartments.

Internalization of FTC-labelled somatostatin was prevented by carrying out the incubation at 4° C. or, alternatively, in the presence of the endocytosis inhibitor phenylarsine oxide, thereby indicating that the internalization process was endocytic in nature. In both conditions, labeling remained confined to the cell surface as confirmed by confocal microscopic optical sectioning and by the fact that the labeling was totally abolished by a two-minute wash in acid solution (pH 4).

Somatostatin internalization proved to be receptor-dependent, as demonstrated by the lack of either surface labeling (at 4° C.) or intracytoplasmic labeling (at 37° C.) when the experiments were carried out in the presence of a thousand-fold excess of non-fluorescent D-Trp⁸ !-somatostatin.

4. Confocal Microscopic Visualization of Somatostatin Receptors: Internalization of α-BODIPY-Somatostatin and FTC-Somatostatin in COS Cells Expressing sstr1 or sstr2a Receptors

Incubation of COS cells transiently transfected with cDNA encoding sstr1 or sstr2 somatostatin receptor sub-types with 5 nM of either a-Bodipy D-Trp8! somatostatin resulted in an intense fluorescent labeling of the cells. Approximately 50% of the cells were labelled with fluorescent somatostatin, and thus corresponded to those cells expressing somatostatin receptors (as verified by labelling with antibodies raised against somatostatin receptors). Serial confocal microscopic optical sectioning revealed that this labeling was intracytoplasmic and in the form of endosome-like particles.

Internalization of fluorescent somatostatin was prevented by concomitant incubation with the endocytosis inhibitor, phenylarsine oxide, in which case the labeling remained confined to the cell surface. Binding and internalization of both ligands were totally prevented when the incubation was carried out in the presence of a thousand-fold excess of non-fluorescent D-Trp⁸ ! somatostatin.

5. Internalization of a-Bodipy- D-Trp⁸ ! Somatostatin-14 and a-FTC- D-Trp⁸ ! Somatostatin-14 in Slices of Rat Hypothalamus

Superfusion of rat brain slices with 2-20 nM of either α-FTC-labelled somatostatin or α-Bodipy-labelled somatostatin for 45 minutes at 37° C. resulted in selective labeling of neuronal perikarya and dendrites dispersed across several brain regions. Labeling observed with either ligand was no longer apparent when the incubation was carried out at 4° C. or in the presence of a thousand-fold excess of non-fluorescent D-Trp⁸ ! somatostatin.

In the arcuate nucleus and lateral tuberal nucleus of the hypothalamus, the grouping of fluorescent somatostatin-labeled cells was reminiscent of that of neurons previously found to bind and express somatostatin receptors. At high magnification using the confocal microscope described above, the fluorescent signal took the form of small intensely fluorescent granules, suggesting an endocytotic compartmentalization of internalized ligand molecules comparable to that seen in transfected cell lines.

Other embodiments are within the scope of the following claims.

    __________________________________________________________________________     SEQUENCE LISTING                                                               (1) GENERAL INFORMATION:                                                       (iii) NUMBER OF SEQUENCES: 5                                                   (2) INFORMATION FOR SEQ ID NO:1:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 5 amino acids                                                      (B) TYPE: amino acid                                                           (C) STRANDEDNESS: Not Relevant                                                 (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: protein                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:                                        PhePheTrpLysThr                                                                15                                                                             (2) INFORMATION FOR SEQ ID NO:2:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 6 amino acids                                                      (B) TYPE: amino acid                                                           (C) STRANDEDNESS: Not Relevant                                                 (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: protein                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:                                        AlaGlyCysLysAsnPhe                                                             15                                                                             (2) INFORMATION FOR SEQ ID NO:3:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 4 amino acids                                                      (B) TYPE: amino acid                                                           (C) STRANDEDNESS: Not Relevant                                                 (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: protein                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:                                        PheThrSerCys                                                                   (2) INFORMATION FOR SEQ ID NO:4:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 14 amino acids                                                     (B) TYPE: amino acid                                                           (C) STRANDEDNESS: Not Relevant                                                 (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: protein                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4:                                        AlaGlyCysLysAsnPhePheTrpLysThrPheThrSerCys                                     1510                                                                           (2) INFORMATION FOR SEQ ID NO:5:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 5 amino acids                                                      (B) TYPE: amino acid                                                           (C) STRANDEDNESS: Not Relevant                                                 (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: protein                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:5:                                        PheTrpLysThrPhe                                                                15                                                                             __________________________________________________________________________ 

What is claimed is:
 1. A compound of the formula: ##STR4## wherein R₁ is a light-emitting moiety and R₂ is a somatostatin-based peptide comprising a first amino acid sequence --Phe--Phe--Trp--Lys--Thr-- (SEQ ID NO:1) or --Phe--Phe--D--Trp--Lys--Thr--, said peptide being linked at a first amino acid position to (C--X), wherein (C--X) is selected from the group consisting of C═O, C═S, CHOH, C═C═O, C═NH, CH₂, CHOH, CHOR, CNR, CH--R, CR₃ R₄, and COR₃ OR₄ wherein R, R₃, and R₄ are alkyl moieties or substituted alkyl moieties, wherein said compound exhibits substantial biological activity in the presence of a receptor having affinity for somatostatin-based peptides.
 2. The compound of claim 1, wherein said first amino acid sequence --Phe--Phe--Trp--Lys--Thr-- (SEQ ID NO:1) or --Phe--Phe--D--Trp--Lys--Thr-- is located at positions 7-11 in said somatostatin-based peptide.
 3. The compound of claim 1, wherein said first amino acid position comprises the N-terminus of said somatostatin-based peptide.
 4. The compound of claim 1, wherein said somatostatin-based peptide further comprises the amino acid sequence --Ala--Gly--Cys--Lys--Asn--Phe-- (SEQ ID NO:2).
 5. The compound of claim 4, wherein the amino acid sequence --Ala--Gly--Cys--Lys--Asn--Phe-- (SEQ ID NO:2) comprises the N-terminus of said somatostatin-based peptide.
 6. The compound of claim 5, wherein said --Ala-residue of said somatostatin-based peptide is attached to (C--X).
 7. The compound of claim 1, wherein said somatostatin-based peptide further comprises the amino acid sequence --Phe--Thr--Ser--Cys-- (SEQ ID NO:3).
 8. The compound of claim 7, wherein said amino acid sequence --Phe--Thr--Ser--Cys-- (SEQ ID NO:3) comprises the C-terminus of said somatostatin-based peptide.
 9. The compound of claim 1, wherein said somatostatin-based peptide further comprises the amino acid sequences --Ala--Gly--Cys--Lys--Asn--Phe-- (SEQ ID NO:2) and --Phe--Thr--Ser--Cys-- (SEQ ID NO:3).
 10. The compound of claim 9, wherein said somatostatin-based peptide is somatostatin-14 or D-Trp⁸ -somatostatin-14.
 11. The compound of claim 1, wherein said light-emitting moiety is selected from the group consisting of Bodipy, fluorescein, FTC, Texas red, phycoerythrin, rhodamine, carboxytetra-methylrhodamine, DAPI, indopyras dyes, Cascade blue, coumarins, NBD, Lucifer Yellow, propidium iodide, and derivatives thereof.
 12. The compound of claim 1, wherein (C--X) is C═O or C═S.
 13. The compound of claim 1, wherein said compound is a pharmaceutically acceptable salt or complex thereof.
 14. A method for labelling a receptor having an affinity for a somatostatin-based peptide by contacting said receptor with the compound of claim
 1. 15. A light-emitting compound of the formula: ##STR5## wherein R₁ is a light-emitting moiety and R₂ is a somatostatin-based peptide comprising a first amino acid sequence --Phe--Phe--Trp--Lys--Thr-- (SEQ ID NO:1) or --Phe--Phe--D--Trp--Lys--Thr--, said peptide being linked at a first amino acid position to (C--X), wherein (C--X) is selected from the group consisting of C═O, C═S, CH(OH), C═C═O, C═NH, CH₂, CHOH, CHOR, CNR, CH--R, CR₃ R₄, and COR₃ OR₄ wherein R, R₃, and R₄ are alkyl moieties or substituted alkyl moieties,wherein said light-emitting compound exhibits substantial biological activity in the presence of a receptor having affinity for somatostatin-based peptides, said light-emitting compound being selected from a mixture including said light-emitting compound and secondary compounds having biological activities less than 0.25% of the biological activity of R₂ -H in the presence of a receptor having affinity for somatostatin-based peptides.
 16. The light-emitting compound of claim 15, wherein said selection is accomplished by exposing said mixture to a receptor recognizing somatostatin-based compounds, and then selectively isolating said light-emitting compound exhibiting substantial biological activity in the presence of a receptor having affinity for somatostatin-based peptides from said mixture.
 17. The light-emitting compound of claim 16, wherein said light-emitting compound having substantial biological activity in the presence of a receptor having affinity for somatostatin-based peptides is selected using high-pressure liquid chromatography.
 18. The light-emitting compound of claim 15, wherein said first amino acid sequence --Phe--Phe--Trp--Lys--Thr-- (SEQ ID NO:1) or --Phe--Phe--D--Trp--Lys--Thr-- is located at positions 7-11 in said somatostatin-based peptide. 